Anatomic and Non-Anatomic Shoulder Replacement
In the field of shoulder arthroplasty, there are two general and somewhat competing points of view regarding the state of the patient's anatomy. From the point of view of some clinicians, it is desirable to aim for restoration of the native anatomy through use of prosthetic shoulder components that are shaped in a manner that is anatomically correct, particularly with regards to the shape of the prosthetic humeral head. For others, the higher objective is to aim for adapting and balancing the existing soft tissues, particularly the rotator cuff and musculature, with the shape and orientation of the replacement humeral head, even if the shape of the prosthetic head is not anatomically correct.
Shoulder arthroplasty typically requires removal of the entire head of the humerus bone, commonly at or below the anatomical neck, to accommodate insertion of a prosthesis, typically in the form of a head-bearing elongated shaft (referred to herein as a stem), into the diaphysis of the humerus, and in alternate approaches a stemless system includes a cage or other support structure that is not elongate. The head portion of the prosthesis provides an articulation surface that cooperates with an opposing articulation surface, the glenoid, which is on the boney portion of the scapula. In some instances, the head and stem of the prosthesis are unitary, while in other instances, the head and stem are provided as discrete components that are engageable by a variety of means, such as a male taper and female receiver. Within the art, there is a wide array of choices with respect to stem features, in terms of length, width, taper, and dimensions, as well as shape and texture. Likewise, there is a wide array of choices with respect to humeral head shape, dimensions, pitch, and the like.
Stemless shoulder prostheses are also known in the art. Such prostheses are offered currently in Europe commercially, and are under investigation in the United States. The stemless systems are considered anatomically accurate by nature due to the generally greater ease of component positioning as compared with systems that use stems. The stemless systems utilize spherical humeral heads in all variations. The stemless systems are particularly desirable because they involve less invasive boney operations, and because the surgical technique itself is not as technically demanding, since the final position of the prosthetic head is not constrained by the long axis of the bone due to the short length of the prosthesis. Fixation is offered in a variety of keeled, caged, caged-pegged configurations. However, poor bone quality presents concern for long-term durability of stemless arthroplasty, and poor bone quality is considered a contraindication for use of a stemless prosthesis. This limits the utility of stemless implants typically to patients who are young, since elderly patients—who are most often in need of joint replacement—often have osteopenic bone and are thus excluded from the possibility of stemless shoulder arthroplasty.
The anatomic approach involves restoration of the humeral head to its pre-diseased state, with utilization of spherical humeral head components with proportional diameter and thickness. In contrast, the non-anatomic approach involves humeral head replacement with soft-tissue balancing of the rotator cuff utilizing spherical humeral head components of varying thicknesses. Generally, within the art, reverse shoulder arthroplasty is considered non-anatomic shoulder replacement because the native glenoid side of the shoulder is converted to a sphere to mimic the humerus (glenosphere), while the humeral side is converted to mimic a glenoid (typically through replacement of the humeral head with a cup shaped implant).
Desired features of anatomic implants include replication of humeral neck angle, version, and posterior and medial offset. In the current art, stemmed arthroplasty systems are the most prevalent, and essentially all stemmed arthroplasty systems use spherical humeral heads. The conventional belief is that roughly one-third of a sphere is the most anatomically correct shape of the current offerings. Regardless of head size, the ratio of the head height to the radius of curvature is about 3:4. Clinical outcomes in patients who have received anatomically correct prostheses are generally regarded as superior when compared to soft-tissue balancing techniques using non-anatomically shaped (i.e., anatomically incorrect) prostheses.
Whether or not an implant is anatomically correct, some implants in the art are designed to be usable in either a standard to a reverse configuration. Typically, within the art, convertible implants allow the surgeon to convert by removing the standard prosthetic head from the stem and replacing the head with a cup (to mimic the glenoid) (examples within the art include convertible shoulder arthroplasty systems by Biomet, Zimmer, Tornier, Exactech). With such prostheses, the cup sits on top of the bone cut rather than being recessed within the bone. A disadvantage of this technique and prosthesis design is that the humerus becomes overlengthened or distalized, predisposing the patient to nerve stretch injury, joint stiffness, and acromial fracture. Thus, while these convertible systems offer the benefit of a less invasive reoperation, the tradeoff is increased risk of surgical complications and inferior biomechanical outcomes, all of which are due to the increased height of the implant that result from placement of the cup above the bone cut. This is particularly true with respect to reverse shoulder revisions when compared to primary reverse shoulder arthroplasty that is achieved with a reverse-specific implant where the cup is recessed into the proximal humerus bone (examples within the art of primary reverse shoulder arthroplasty systems include those by DJO Surgical, DePuy, and Tornier). Arm lengthening, nerve palsies, joint instability, impingement, joint stiffness, acromial fractures, and difficulty with prosthesis conversion that ultimately leads to stem extraction and bone fracture are all examples of undesirable clinical outcomes resulting from current convertible and primary arthroplasty systems.
Most reverse shoulder arthroplasty systems are designed to deliberately shift the rotational center of the joint in order to take what is believed to be best advantage of the remaining musculature by tensioning the deltoid to compensate for loss of rotator cuff function. The approach yields a distal shift of the arm/humerus (i.e., towards the direction of the patient's feet). This distal shift is achieved through an increase in the overall length of the humerus through the height of the implant beyond the cut line of the humeral head. While there are perceived advantages to this approach, known problems that come with increased distalization of the arm include 1) acromial/scapular fracture, and 2) nerve injury from the stretch on the nerves. Indeed, while some experts may tout the advantages of increasing deltoid tension, others report that “ . . . an increase in passive tension of the deltoid on the acromion, can lead to fatigue, stress, or complete fracture [Hamid N, et al. Acromial Fracture After Reverse Shoulder Arthroplasty. Am J Orthop. 2011.40(7):E125-E129]. Werner et al reported a 7.3 incidence of scapular fracture in revision cases, and a 6.3% incidence during primary arthroplasty [Werner C M, et al. Treatment of painful pseudo-paresis due to irreparable rotator cuff dysfunction with the Delta III reverse-ball-and-socket total shoulder prosthesis. J Bone Joint Surg Am. 2005.87:1476-86]. Others have reported a 7.7% incidence of neuropraxia during revision reverse shoulder arthroplasty [Total Reverse Shoulder Arthroplasty: European Lessons and Future Trends. Seebauer L. Am J Orthop. 2007.36(12 Supplement):22-28.]. The high incidence of nerve injury is probably due to the stretch on the brachial plexus nerves that occurs as the humerus is lengthened. Especially in patients with stiff, contracted shoulders, it is not advisable to over-lengthen the arm. In view of these undesirable clinical effects that derive from the mechanical lengthening of the bone, there is a need to provide an arthroplasty system that is specifically designed to avoid distalization.
Yet another challenge in the art is the absence of anatomically correct head articulation surfaces. It is known that the native anatomical shape of the humeral head is not spherical, but elliptical (i.e., where the cross section of the humeral head has a radius of curvature in the superior to inferior dimension that is greater than the radius of curvature of the cross section in the anterior to posterior dimension). Recent research has shown that a prosthetic humeral head having a cross sectional shape adjacent to the bone cut that is elliptically-shaped and a generally spherical center point would theoretically allow a patient to have improved shoulder range of motion and function postoperatively. However, because the center of rotation of the humeral head is offset from the long axis of the humeral bone, it has been impractical for any shoulder implant company to create a prosthesis with an elliptically-shaped prosthetic humeral head. Merely coupling an elliptically-shaped head with a traditional stemmed prosthesis design would present difficulties accounting for the surgeon's need to simultaneously achieve the proper head size, correct rotational orientation of the elliptical head, and the proper amount of superior to inferior and anterior to posterior offset relative to the stem.
Moreover, in many shoulder surgeries, only the humeral portion of the joint is replaced while the native glenoid is left intact, presenting a challenge of matching the articulating surface of the head prosthetic with the native articulating surface of the glenoid. This challenge is not present in total arthroplasty, where both the humeral and the glenoid portions are replaced with prosthetics. Ideally, a shoulder arthroplasty system would provide a wide range of head choices and offsets to most precisely match the patient's native anatomy. With such a system, a near perfect match could be achieved in a hemi-arthroplasty, and in if the system were modular, could be adapted in a revision to provide an ideal match if the shoulder is converted to either a total arthroplasty or to a reverse shoulder arthroplasty. The current art does not provide such modular systems, thus, to accomplish the desirable offsets with traditional stem designs, whether using spherical or elliptical heads, it would be necessary to stock an essentially infinite inventory of prosthetic heads and/or stems with variable offsets for achieving the desired shape, size and positioning, which is, of course, economically impractical.
Another challenge in joint replacement is the general requirement for complete implant removal in the instance where a corrective or revision surgery is needed with a primary arthroplasty system. A common feature among the shoulder arthroplasty devices in the art is that they are typically designed for a single use, and typically cannot be repurposed in a later surgery on the same patient. That is to say that any post implantation procedure which the patient may require due to further bone or soft tissue deterioration, such as a revision or conversion to a reverse configuration, typically requires a bony procedure wherein all or a portion of the implanted prosthesis must be removed from bone in order to allow implantation of a new device. It is well known that in a percentage of initial shoulder arthroplasty cases, the patient will require revision surgery due to device failure, infection, or further degeneration of the bone or soft tissues of the joint. In some specific situations, the revision will require conversion of the humeral side of the joint from a standard implant to a reverse implant. It is desirable, though typically not possible, to avoid any bony procedure during revision cases because there is a high risk of humeral fracture and/or bony destruction when the surgeon attempts to remove a well-fixed humeral component from the humerus. It is desirable to advance the art with devices that achieve structural stability of an implant within the bone while retaining the ability to remove the device without bone fracture or catastrophic loss of bone during removal.
The objective of implant stability is addressed, in the context of long bones, through implant length, proximal diameter, and material selection and surface treatment that can enhance bony ingrowth on the implant. In the art of shoulder arthroplasty, there are a variety of short-stemmed and stemless devices that have implant surface features that encourage bony ingrowth and implant dimensions that are intended to achieve stability. While these features are helpful to encourage securement within bone, they are developed based on averages within a broad patient population, for example in terms of proximal humerus head and diaphysis dimensions and contribute to some of the other challenges of arthroplasty in that they provide only a limited range of possible device configurations and features for achieving bony fixation. And it is a well-known problem that removal of a prosthesis component that is well fixed in the bone is made more difficult when the structural features of implant components limit the surgeon's ability to apply surgical instruments such as an osteotome to free the prosthesis from the bone, especially in the metaphyseal and diaphyseal regions. It is the very structural elements that provide the opportunity for enhanced fixation that also lead to significant bone damage and loss in the likely event that a revision is needed. The art presently lacks arthroplasty implants with features that enable achievement of bony fixation and enable removal of components for revision to minimize bone loss while enabling the repurposing of the primary implants for alternate use.
A need exists to provide a humeral prosthesis that is designed to be modular and adaptable to enable a closer approximation of native anatomical fit for a broader range of patients rather than a patient population. Further, there is a need for a device that mitigates the problems associated with height position of a prosthesis in the humerus bone at the time of the index procedure and/or a revision surgery so that distalization of the humerus is avoided if conversion to a reverse shoulder arthroplasty is required. And there is a need for devices that are optimized for proximal bony ingrowth and distal (diaphyseal) stability to achieve short and long-term device stability while retaining the ability to revise and possibly remove the implant without catastrophic bone effects. While some devices and device features exist within the art that are designed to protect against humeral bone loss in revision surgeries, there remains a need for a system that enables replacement or conversion of a humeral prosthesis without the requirement for bony procedure or at least minimal need for removal of implant from within the bone. To address needs in the art, including the several needs identified, this disclosure provides a system that is modular and convertible and optimized achieve closer approximation of a patient's native anatomy, including avoidance of arm distalization, avoidance of surgery-related bone loss, while enabling a wider range of options for matching anatomy on during the index procedure as well as during surgical revision.